Electrochemistry in Stress Corrosion Cracking of Structural Al alloys

The effect of constant bending load on corrosion and repassivation of high strength 2024-T3 and 7075-T6 Al alloys in NaCl solutions was investigated by means of single cycle anodic polarization and galvanic corrosion experiments. Bending loads up to the yield strength of a given alloy were applied using four point bent beam (4PPB) stress corrosion test specimens. A critical electrochemical condition in correspondence with the onset of micro-plasticity is identified from the repassivation behavior. The estimated reaction order with respect to Cl-suggests proton consumption/generation involved in crack nucleation and propagation. Galvanic corrosion experiments provided evidence of stress assisted intergranular dissolution, being enhanced in the presence of galvanic coupling with steel. The less evident effect of bending load on the repassivation and galvanic corrosion of Al2024-T3 appears to be determined by the inherent microstructural corrosion. Nonetheless, stress effects (tensile, compressive, and residual) importantly influence the electrochemical behavior of both alloys. Overall results emphasize the interdependence between the local aggressive environment, the electrochemical event and the mechanical state in the stress corrosion cracking of Al alloys.